Chirped amplitude modulation ladar

ABSTRACT

An imaging method and apparatus using an unmodulated pulsed laser with a chirp modulated receiver is provided for producing 3D plus intensity imagery of targets in heavily cluttered locations The apparatus includes a laser for emitting a laser beam and synchronizing a receiver to receive a reflected laser signal and transform the reflected laser signal into a displayable image that includes intensity information.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the United States Government,

BACKGROUND

Laser detection and ranging (ladar) may be used in a military contextfor elastic backscatter light detection and ranging (lidar) systems. Theacronym LADAR is usually associated with the detection of hard targets,while the acronym LIDAR is usually associated with the detection ofaerosol targets. However, there has been no real standard on their useand both acronyms may be used interchangeably to describe the same laserranging system. A ladar system is similar to a radar system with theexception that a much shorter wavelength of the electromagnetic spectrumis used, typically in the ultraviolet, visible, or near infraredspectrums. With a radar system, it is possible to image a feature orobject of about the same size of the wavelength or larger. Due todiffraction limits, the laser radiation is easier to collimate thanmicrowave radiation given realistic aperture constraints. This gives acompact ladar the ability to image a target with at a high spatialresolution.

In order for a ladar system target to reflect a transmittedelectromagnetic wave, an object needs to produce a dielectricdiscontinuity from its surroundings. At radar frequencies, a metallicobject produces a dielectric discontinuity and a significant specularreflection. However, non-metallic objects, such as rain and rocksproduce weaker reflections, and some materials may produce no detectablereflection at all, meaning some objects or features are effectivelyinvisible at radar frequencies. In a military context, metallic objectsmay be disguised by use of a non-metallic covering material.

Lasers provide one solution to this problem regarding non-metallicdetection. The beam densities and coherency of lasers are excellent.Moreover, the wavelengths are much smaller than can be achieved withradio systems, and range from about 10 μm to around 250 nm. At suchwavelengths, the waves are reflected very well from small objects suchas molecules and atoms. This type of reflection is called diffuse“backscattering.” Both diffuse and specular reflection may be used fordifferent ladar applications.

The Army Research Laboratory has demonstrated the capability of itspreviously patented chirped amplitude modulation (AM) ladar technique toproduce high-resolution, range-resolved intensity imagery (3D+intensity)of targets in heavy clutter, under dense foliage canopy, and obscured bycamouflage nets. This technique is covered in U.S. Pat. No. 5,877,851for “Scannerless Ladar Architecture Employing Focal Plane DetectorArrays and FM-CW Ranging Theory” and in U.S. Pat. No. 5,608,514 for“High-Range Resolution Ladar,” which are hereby incorporated byreference. These previously patented techniques use a modulatedcontinuous wave (CW) laser as an illumination source and a photondetection receiver. For some applications, a high-peak powerillumination source is required to satisfy the design requirements,which is not easily achieved with a CW source. For those applications,it is advantageous to use a short-pulse laser as an illumination sourcedue to their high-peak powers and commercial availability. However,since the duration of the laser pulse is extremely short, it is notfeasible to chirp modulate the laser intensity as is required with thecurrent ladar architecture Therefore, a modification to the existingranging technique is required

SUMMARY

Embodiments of the present disclosure provide apparatuses and methodsfor using Chirped Amplitude Modulation Ladar. Briefly described, oneembodiment of the apparatus, among others, comprises: a laser sourceconfigured to emit an unmodulated pulsed laser beam, a reflection ofwhich produces an optical signal; a receiver configured to receive theoptical signal; a modulator configured to modulate a gain of thereceiver to produce a modulated optical signal; an integrator configuredto determine a state of the modulated optical signal; and a detectioncircuit adapted to measure the amplitude of the state of the modulatedoptical signal

One embodiment of such a method, among others, can be broadly summarizedby the following steps: triggering a laser to emit an unmodulated laserbeam; modulating the gain of a receiver; receiving, at the modulatingreceiver, a reflected laser beam signal associated with the laser beamemission; and determining at least one characteristic associated withthe reflected laser beam signal.

Other systems, methods, features, and/or advantages of the presentdisclosure will be or may become apparent to one with skill in the artupon examination of the following drawings and detailed description. Itis intended that all such additional systems, methods, features, and/oradvantages be included within this description, be within the scope ofthe present disclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a top view of an environment in which an exemplary embodimentmay be utilized.

FIG. 2 is a block diagram of an exemplary embodiment of a chirpedamplitude modulation ladar using a short-pulse laser transmitter whichmay be utilized in the environment provided in FIG. 1.

FIG. 3 is a comparison graph of a sampled continuous chirp waveform anda step-chirp waveform for a small number of sample times as used in anexemplary embodiment according to FIG. 2.

FIG. 4 is a flow diagram of a method of using a short-pulse lasertransmitter for chirped amplitude modulation ladar as used in anexemplary embodiment according to FIG. 2.

FIG. 5 is a graph of detected target signals for two individual targetsat positions of 0.25 and 0.5 respectively.

FIG, 6 is a graph of a discrete Fourier transform of the reflectedsignal of the two targets of FIG. 5.

FIG. 7 is a graph of the superposition of the oscillations of the twofrequencies corresponding to the range of each individual target of FIG.5.

FIG. 8 is a graph of the magnitude of the discrete Fourier transform forthe 2-target signal of FIG. 7.

DETAILED DESCRIPTION

Embodiments of an apparatus for chirped amplitude modulation ladardisclosed herein use a short-pulse laser as an illumination source. Theillumination source is used to interrogate the state of the localoscillator waveform applied to any modulatable optical receivercompatible with the laser source and modulation waveform. Theillumination source is not amplitude modulated with the modulationwaveform, which can simplify its implementation and allow the use ofcommercial-off-the-shelf (COTS) lasers. The laser pulse width ispreferably less than one-half the pulse width of the maximum modulationfrequency, and may be as large as several tens of nanoseconds. Thispulse width is compatible with commercially available high-peak powerpulsed lasers.

Several sensors are being developed in the area of long-range detectionin the classification of targets, such as the Laser Illuminated Viewingand Ranging (LIVAR®) imaging sensor and the Cost Effective TargetingSystem (CETS) that operate in the 1.55 micrometer wavelength operation.Both sensors function by triggering the laser to transmit a short pulseof light to illuminate the area of interest. Simultaneously, an imageintensifier is turned on, or gated, for a short period of time after apredetermined amount of delay from the laser pulse trigger. The gatingforms an image on the intensifier tube due to the scattered light fromthe laser and in a range swath proportional to the gate time, roughly 36meters for a 120 ns gate, at a distance from the sensor set by the delaytime. These sensors are useful for detecting and classifying targets inthe open since they provide high resolution imagery with reasonablesignal-to-noise ratio using the intensifier tube as a receiver and thelaser as a controllable illumination source. The sensor has difficulty,however, in cluttered terrain since it cannot separate targets fromclutter objects when they are within the same range swath.

One approach to solving this deficiency is to continue to reduce thegate-width to a few nanoseconds, thus decreasing the probability that aclutter object and the target are in the same range of the swath. Usingthis approach would require the sensor to acquire multiple gated imagesin order to build an effective overall image over a reasonable targetsize, making the gate timing delay difficult to implement. Also,generating such a narrow tube gate is very difficult to implement in apractical system.

Another approach is to modify the system so that the actual time offlight of the laser pulse is measured directly on the focal plane. Thisis the typical detection technique used with single or few detectorranging sensors, such as a range finder or scanned imaging ladar.However, is very difficult to implement in a large array of detectors.There is significant research to extend the state-of-the-art for suchtime of flight focal plane array sensors. However, there are stillsignificant problems such as pixel cross-talk, clock noise, yield, andmanufacturability which need to be addressed.

Another previous approach is to directly apply the ARL patented chirp AMladar technique to the current sensor. To implement this approach, themodulation waveform, such as a continuous chirp signal, would bedirectly applied to the gain of the image intensifier tube and theintensity of a CW laser source. The two signals would then bemultiplied, or mixed, in the detection process to produce an outputsignal whose frequency is proportional to the range of the target.Application of this technique has a number of advantages such as theability to form multiple range cells inside of the current range swathand to image multiple targets in a given pixel. Unfortunately, to fullyimplement this technique requires a modulated high-peak-power laserillumination source, which may be difficult to design.

FIG. 1 depicts an environment in which an exemplary embodiment ofchirped amplitude modulation ladar system 100 may be utilized. Theenvironment includes target 110 surrounded by various non-targetsubjects 120A-F, which may mask target 110 from conventional radardetection systems. Ladar system 100 emits laser pulse 105. After laserpulse 105 reaches target 110, reflection 115 is received at ladar system100 where reflection 115 is processed. Target 110 can be discerned fromnon-target objects 120A-F through processing performed in ladar system100,

FIG. 2 is a simplified block diagram of one exemplary embodiment ofchirped amplitude modulation ladar (ladar system) 100 using ashort-pulse laser transmitter. In this exemplary embodiment, directdigital synthesizer (DDS) 205 is operated in a stepped-frequencymodulation mode for the chirp modulation source in order to modulate thegain of the receiver. However, any other ranging waveforms may beimplemented as discussed in the references above, such as, but notlimited to, continuous and stepped versions of saw-toothed frequencychirp, up-down frequency chirp, or a pseudo-random code. DDS 205 is alsoused to initiate gate and modulation circuitry 250, which gates opticalreceiver 210 on and off to form the range swath. Any device capable ofthe gating function could also be used in conjunction with DDS 205.

Modulatable optical receiver 210 shown in FIG. 2 may be anelectron-bombarded image intensifier tube with a voltage controllableoptical gain for converting the signal received through receiver optics235 into an electrical signal. However, any modulatable optical receivercompatible with laser source 215 and modulation waveform produced bygate and modulation circuitry 250, such as, but not limited to,self-mixing detectors, electro-optic modulator followed by detectorfocal plane array, an avalanche photo-detector array, or aphoton-counting detector array. In this embodiment, the optical receiveris communicatively coupled to an integrating read-out structure 220 tointegrate the signal over the gate width. However, any detection circuitcapable of measuring the signal amplitude can also accomplish the signaldetection. Read-out structure 220 may be comprised of storage 260 anddiscrete Fourier transform module 270 to produce 4-D range-image file280 for display.

Master controller 218 controls trigger circuit 230 which triggersshort-pulse laser 215 in an exemplary embodiment. The laser is emittedthrough transmitter beam shaping optics 225 towards potential objectsfor which some measurement (e.g., distance) is desired. Pulse dectector245 detects the laser pulse emission, and together with the mastercontroller, signal a time-delay to be generated by time-delay generator240. The time delay generated by time-delay generator 240 is used by DDS205 to adjust the output of gate and modulation circuitry 250.

FIG. 3 is an illustrative comparison of a sampled continuous chirpwaveform 300 and a step-chirp waveform 310 for a small number of sampletimes. By sampling the continuous chirp waveform 300 at discrete times305, the continuous chirp 300 can be replaced by a step-chirped waveform310 whose stepped frequencies are equal to the instantaneous frequencyof the continuous chirp at sample times 305. When this is performed, thesignal processing 320 of the sample of intermediate frequency signalderived from either chirped waveform is identical. Embodiments of achirped amplitude modulation ladar may use either chirp waveform;however, for illustrative purposes, it is simpler to explain itsoperation mathematically using the stepped frequency chirped waveform.

FIG. 4 provides a flow diagram 400 for an exemplary embodiment asillustrated in FIG. 2. To begin the acquisition of a ladar, first,calculations are made for the time delay, gate time, chirp frequencydeviation, number of frequency steps, and frequency step size. The timedelay is calculated using the range to the target, r, and the speed oflight, c, by

$t_{d} = {\frac{2R}{c}.}$

The gate time is determined similarly using the required range swath,Rs, by

$t_{g} = {\frac{2R_{s}}{c}.}$

The chirped frequency deviation, Δv, is determined by the rangeresolution, ΔR, using

${\Delta \; F} = {\frac{c}{2\Delta \; R}.}$

The number of frequency steps, N_(steps), required for an unaliasedrange measurement is 2Rs/ΔR and the frequency step size, F_(steps) isΔF/N_(steps).

Once these parameters are determined, in block 410, the DDS may beprogrammed with a sinusoidal frequency F_(start) and the laser pulse isinitiated in block 420. The laser pulse exits the laser cavity and, inblock 425, a sample of the pulse is measured by the pulse detector andconverted to an electrical signal. In block 430, this electrical signal,which is a measure of the exact time of the laser emission, is delayedby t_(d) and used to trigger the DDS and gating circuit. In block 440,the DDS gates or turns-on the electronic-bombarded image intensifiertube for t_(d) and concurrently modulates the gain of the imageintensifier with the sinusoidal signal at frequency F_(start).

Substantially simultaneously, the laser pulse travels out to the targetat range R, is scattered by the target, and a portion of the scatteredsignal is collected by the receiver optics and imaged onto thephotocathode of the image intensifier tube in block 450. The short-pulseof a laser light is detected by the photocathode and converted to anelectrical signal that is amplified in the image intensifier tube by theinstantaneous value of its modulated gain Each detecting element in thefocal plane array then integrates the electrical signal over the gatetime t_(g), amplifies the signal, and converts the signal to a digitalnumber that is then stored in the memory device. After the digital datais stored, the master controller sets the DDS output frequency toF_(start)+F_(step) in block 465 and repeats the process. This iscontinued until, in block 460, the number of steps, N_(steps), necessaryto complete the chirp waveform is determined to have been reached. Inblock 470, the stored data is processed using discrete Fouriertransforms to generate the 3-D plus intensity data set (4-D image), anddisplayed in an appropriate format in block 480. Since the samemodulation waveform is applied to every pixel of the receiver, theranges and intensities of each scatterer within the gate limited rangeswath viewed by each pixel are measured, so that a 3-D plus intensityimage of the scene can be formed using this technique.

As an illustrative case of the embodiment of FIG. 2, a target may beimaged at a range of 5 km over a range swath of 18 m with a rangeresolution of 0.564 m. Using these parameters, t_(d) is calculated at33.3 μs, t_(g) is 120.0 ns, ΔF is 266 MHz, N_(step) is 64, and F_(step)is 4.16 MHz. For this example, a starting modulation frequency of 30 MHzis chosen. From the flow diagram of FIG. 4, to begin an imagecollection, the master controller sets the DDS output frequency to 30MHz and initiates a laser pulse. This laser pulse is detected, convertedto an electrical signal, delayed by the time delay generator by 33.3 μs,and used to trigger the DDS. The DDS gates the tube for 120 ns andsimultaneously modulates the gain of the electron bombarded imageintensifier tube at a 30 MHz frequency,

The target signal is detected by the photocathode, converted to anelectrical signal, integrated in each signal in the read-out structure,amplified, converted to a digital number, and stored in memory. Afterthe digital data is stored, the master controller sets the DDS outputfrequency to 34.16 MHz and repeats the process. This is continued untilall 64-frequency steps necessary to complete the chirp waveform arefinished. The stored data for each pixel over all frequency steps isthen processed using discrete Fourier transforms to generate the 3-Dplus intensity data set, and displayed on a stereo-graphics enabledcomputer screen.

Mathematically, the signals necessary to explain the ladar operation canbe expressed using the following simplified equations: Transmitterfunction: δT; simplified short pulse initiated at time 0 Receiver gainfunction: ½(1+cos(ω_(step)t))*Gate(t); where Gate=1 or 0 Target functionat receiver δ(t−τ), where τ is the round trip delay time for thetransmitter to the target and back to the receiver.

The target signal detected by the image intensifier tube is found bymultiplying the target function and the receiver gain function, which,because of the property of the delta function, reduces to½(1+cos(ω_(step)τ))*Gate(τ). This signal is then integrated by theread-out structure over the gate-width to convert the optical signal toan electrical signal.

When the gating function G(τ) is 1, the signal measured by the receiveris proportional to the instantaneous gain modulation cos(ω_(step)τ) atthe round trip delay time. FIG. 5 is a plot 500 of detected targetsignals 510, 520 for two individual targets at positions of 0.25 and 0.5respectively. Target signals 510, 520 correspond to the gate widthmeasured from the start of the gate for each frequency step using theparameters given in the example above. As with the original chirp AMladar, each target signal oscillates at a frequency proportional to thetarget's range and, because the target signal is multiplied by theoverall gating function; this range is limited to be inside of the rangeswath. As provided in FIG. 6, graph 600 provides a discrete Fouriertransform of the target's signal, which maps the target frequency totarget range inside of the gate, with the targets, 0.25 gate 610 and 0.5gate 620, appearing in the 8^(th) and 16^(th) range cell.

If, instead of a single target illuminated in a pixel, there are twotargets simultaneously illuminated in the same pixel, represented by atarget function δ(t−τ₁)+δ(t−τ₂), the signal detected by the receiver ateach frequency step is¼(1+cos(ω_(step)τ₁))*Gate(τ₁)+½(1+cos(ω_(step)τ₂))*Gate(τ₂) When bothtargets are within the gate time so that both Gate(τ₁) and Gate(τ₂) are1, the signal collected during the chirp will oscillate at thesuperposition of the two frequencies corresponding to each individualtarget's range, as plotted in FIG. 7. Although waveform 700 appearscomplicated when plotted as a function of time. After transforming thesignal to range space by taking the DFT, discrete Fourier transform,both targets are easily separated and detected in the plot of themagnitude of the discrete Fourier transform for the 2-target signaldepicted by waveform 800 shown in FIG. 8.

Processing embodiments of the present disclosure can be implemented inhardware, software, firmware, or a combination thereof. In the preferredembodiment(s), processing is implemented in software or firmware that isstored in a memory and that is executed by a suitable instructionexecution system. If implemented in hardware, as in an alternativeembodiment, the processing can be implemented with any or a combinationof the following technologies, which are all well known in the art: adiscrete logic circuit(s) having logic gates for implementing logicfunctions upon data signals, an application specific integrated circuit(ASIC) having appropriate combinational logic gates, a programmable gatearray(s) (PGA), a field programmable gate array (FPGA), etc.

The flow chart of FIG. 4 shows the architecture, functionality, andoperation of a possible implementation of the processing software. Inthis regard, each block represents a module, segment, or portion ofcode, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat in some alternative implementations, the functions noted in theblocks may occur out of the order noted in FIG. 4. For example, twoblocks shown in succession in FIG. 4 may in fact be executedsubstantially concurrently or the blocks may sometimes be executed inthe reverse order, depending upon the functionality involved,.

The processing program, which comprises an ordered listing of executableinstructions for implementing logical functions, can be embodied in anycomputer-readable medium for use by or in connection with an instructionexecution system, apparatus, or device, such as a computer-based system,processor-containing system, or other system that can fetch theinstructions from the instruction execution system, apparatus, or deviceand execute the instructions. In the context of this document, a“computer-readable medium” can be any means that can contain, store,communicate, propagate, or transport the program for use by or inconnection with the instruction execution system, apparatus, or device.The computer readable medium can be, for example but not limited to, anelectronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, device, or propagation medium. Morespecific examples (a nonexhaustive list) of the computer-readable mediumwould include the following: an electrical connection (electronic)having one or more wires, a portable computer diskette (magnetic), arandom access memory (RAM) (electronic), a read-only memory (ROM)(electronic), an erasable programmable read-only memory (EPROM or Flashmemory) (electronic), an optical fiber (optical), and a portable compactdisc read-only memory (CDROM) (optical). Note that the computer-readablemedium could even be paper or another suitable medium upon which theprogram is printed, as the program can be electronically captured, viafor instance optical scanning of the paper or other medium, thencompiled, interpreted or otherwise processed in a suitable manner ifnecessary, and then stored in a computer memory. In addition, the scopeof the present disclosure includes embodying the functionality of thepreferred embodiments of the present disclosure in logic embodied inhardware or software-configured mediums

It should be emphasized that the above-described embodiments of thepresent disclosure, particularly, any “preferred” embodiments, aremerely possible examples of implementations, merely set forth for aclear understanding of the principles of the disclosure Many variationsand modifications may be made to the above-described embodiment(s) ofthe disclosure without departing substantially from the spirit andprinciples of the disclosure. All such modifications and variations areintended to be included herein within the scope of this disclosure andthe present disclosure and protected by the following claims.

1. An apparatus comprising: a laser source configured to emit an unmodulated pulsed laser beam, a reflection of which produces an optical signal; a receiver configured to receive the optical signal; a modulator configured to modulate a gain of the receiver to produce a modulated optical signal; an integrator configured to determine a state of the modulated optical signal; and a detection circuit adapted to measure the amplitude of the state of the modulated optical signal.
 2. The apparatus of claim 1, further comprising: a pulse detector configured to detect a laser emission; and a time delay generator configured to generate a time delay based on the detected laser emission, the time delay being presented to the modulator.
 3. The apparatus of claim 2, further comprising: a trigger circuit configured to trigger the laser and the modulator concurrently.
 4. The apparatus of claim 1, wherein the trigger circuit is configured to trigger the laser to emit the unmodulated laser beam.
 5. The apparatus of claim 1, wherein the modulator comprises a direct digital synthesizer.
 6. The apparatus of claim 5, wherein the direct digital synthesizer is operated in one of a stepped frequency modulation mode, a continuous saw-toothed frequency chirp mode, a stepped saw-toothed frequency chirp mode, up-down frequency chirp mode, and a pseudo-random mode.
 7. The apparatus of claim 1, wherein the receiver comprises an optical receiver.
 8. The apparatus of claim 7, wherein the optical receiver comprises an electron bombarded image intensifier tube with a voltage controllable optical gain.
 9. The apparatus of claim 1, wherein the receiver comprises at least one of a self-mixing detector, an electro-optic modulator followed by a detector focal plane array, an avalanche photodetector array, and a photon-counting detector array.
 10. A method comprising: triggering a laser to emit an unmodulated pulsed laser beam; modulating the gain of a receiver; receiving, at the modulating receiver, a reflected laser beam signal associated with the laser beam emission; and determining at least one characteristic associated with the reflected laser beam signal.
 11. The method of claim 10, further comprising detecting the laser beam emission, wherein the modulating of the gain of the receiver is based on the detecting of the laser beam emission.
 12. The method of claim 10, further comprising displaying a 4-D image associated with the at least one characteristic associated with the reflected signal.
 13. The method of claim 10, further comprising determining if the laser beam emission is complete after receiving the reflected laser beam signal.
 14. The method of claim 13, further comprising: if the laser beam emission is not complete, triggering the laser again to emit an additional unmodulated laser beam; detecting the additional unmodulated laser beam; generating an additional receiver modulation waveform based on the detected additional laser beam emission; receiving an additional reflected laser beam signal; and determining at least one characteristic associated with the detected additional laser beam signal.
 15. The method of claim 14, further comprising displaying a 4-D image associated with the at least one characteristic associated with the detected signal.
 16. The method of claim 13, further comprising: if the laser beam emission is complete then generating an image associated with the at least one characteristic associated with the reflected signal.
 17. The method of claim 16, further comprising performing a transform to generate the image.
 18. The method of claim 15, further comprising displaying a 4-D image associated with the at least one characteristic associated with the reflected signal. 